Ocean Tides

When the water levels go up and down, these are the tides.  The tides of the ocean are caused by how the Earth’s gravity is pulled by the Sun, the moon and how the Earth rotates.

ocean tides create

How Does A Tide Cycle?

When the moon moves around the Earth and when the sun changes positions, this is when the tide cycles.  Since the water is always moving, the level of the sea is always going up or down or rising or falling.

The tide cycle goes like this:

  • The sea level rises
  • A high tide comes
  • The sea level lowers
  • Low tide comes
  • Repeat

Moon & Ocean Tides

The tide cycle happens either one time or two times a day and it depends on where the moon is.  There are two types of tides diurnal and semidiurnal.

Diurnal Tides

A diurnal tide is a tide that happens only one time a day.

Semidiurnal Tides

When a tide happens twice a day, this is called a semidiurnal tide.

Tide Cycle Time

The tide cycle is longer than a day because the way that the moon rotates in the same direction of the Earth, it causes it to be longer.  Instead of 24 hours, the tide cycle is 24 hours and 50 minutes, almost an hour more than a day.

Ocean tides

How Does the Moon Affect the Tide?

The moon has a bigger impact on the tide than the sun does.  Since the moon has a lot of gravity, it can cause a high tide to happen on both sides of the Earth when the Earth is right below the moon.  When this happens, it is called a sublunar tide.

When there is a tide on both sides of the opposite side of the Earth, this is called an antipodal tide.

Low Tides

When there is a low tide, it is found on the side of the Earth that is 90 degrees away from where the moon is.


When the water in the sea is always rising or falling, the water is going to the ocean or coming from the ocean.  When this happens, it causes the water to have tidal currents.  There are different types of currents such as:

  • Slack Water Currents
  • Ebb Currents
  • Flood Currents

When tidal currents happen, it causes water from the ocean to flow into bays and harbors.

Slack Water Currents

When there is a high tide or a low tide, there is no current.  When this happens, it is called a slack water current because there is no current at that time.

Ebb Currents

When there is low tide and the water is lowering, the water is moving towards the ocean and away from the land and this is called an Ebb Current.

Flood Current

When water is moving further away from the ocean and closer to the shoreline, this is called a Flood Current.  This happens when the sea level is moving to high tide.


There is a difference between sea level and range when there is a high or a low tide.  The tidal range can change depending on the position of the moon or the position of the sun.  This can also change due to the geography of the shore.

When the water is near the shore, it can have a much higher tidal range than in the middle of the ocean.  In the middle of the ocean the range might be around 2 or 3 feet and then near the shore it can be as high as 40 feet.

One of the biggest tidal ranges is located on the shore of the Bay of Funday.  This is located in Canada and can have a tidal range more than 40 feet.

When there is a large tidal range, the land where the waves are hitting can be affected by the force of the waves.  When the tidal range is low it means that the energy of the tide will be in the same place.


There are many different types of tides including:  neap tides, diurnal tides, semidiurnal tides, high tides, low tides and spring tides.

Neap Tides

A neap tide is when the tidal range is at the lowest level.  This happens when the moon is three quarters or when there is a first moon.

Diurnal Tides

Diurnal tides are when there is only one low and one high tide that day.

Semidiurnal Tides

Semidiurnal tides happen when there are two low and two high tides in a day.

High Tide

A high tide is when the sea level is the highest.

Low Tide

A low tide is when the sea level is at the lowest.

Spring Tide

A spring tide happens when the moon and the sun are perfectly aligned.  When this happens, there is a large tidal range and the highest high tide and the lowest low tide that happens.

Tidal Power

Tidal power uses the tides of the ocean to make energy.  This type of power is called a clean energy source because it does not use burning or any added chemicals to make the energy.  Tidal energy comes from tides that move when gravity from the moon and how the Earth spins makes the tides move.  When the tides move, this can create a lot of energy.

Is Tidal Power a Renewable Source?

Tidal power is a renewable energy source because there will always be waves and tides.  Renewable means that it cannot be used up and it can always be replaced.  Using tidal energy cannot use up all of the water.

How Do Tides Produce Energy?

There are three different ways that people can get energy from tides.  These include Tidal Turbines, tidal barrages, and tidal fences.

Tidal Turbines

Tidal turbines can be put in any place where there is strong water that moves.  These are turbines that are individual, and they can take the current of the water and make energy.

Tidal Barrages

Tidal barrages are similar to dams.  There is a reservoir that fills up when the tide is high and when the tide gets low, the barrage lets the water out.  The water goes in and out and this causes the turbine blades to spin.  When the blades spin, this makes energy.

Tidal Fences

Tidal fences are smaller than the tidal barrages and they are usually made between two pieces of land.  When the tide moves in and out, it causes the turbine blades to spin.  When the turbine blades spin, this causes there to be electricity.

Where Did the Idea of Tidal Power Come From?

In the year 1940, a man named Yoshio Masdua began to experiment with different types of wave and tide energy.  He wanted to find out if there was a way to use moving water to make energy.  He found that tidal power can be a renewable energy source.

In the older days, tides were used as a power source to help turn wheels and to help grind different kinds of grain.

What is the Problem with Tidal Power?

Tidal power is expensive to use.  It is expensive to find technologies that can collect energy from tides.

In order for tidal power to work, many machines and different things have to be planted.  This cost a lot of money and that is why there is very limited power that is taken from tides.

Another problem with tidal power is that it can cause problems in the environment.  When different technologies are put into the water, it can cause it to be hard for fish to migrate.  The machines can also harm the ocean animals.

More Facts About Ocean Tides:

  • Each month there are two Neap Tides.
  • Each month there are two Spring Tides.
  • A low and high tide are usually around 6 hours apart.
  • Weather can affect the tide.
  • Tidal energy can help to make electricity.

What Did You Learn?

  • What are ocean tides?  Ocean tides are when the sea level rises and falls.
  • What causes ocean tides?  Ocean tides are mostly caused by where the moon and the sun are and the rotation of the Earth.  Ocean tides are also caused by gravity that is pulled from the moon and the sun.
  • What is the difference between a low and high tide?  A low tide is when the water is at the lowest and a high tide is when the water is at the highest.
  • What is tidal power or tidal energy?  Tidal power or tidal energy is when the tides are used to help make energy such as electricity.
  • What is the difference between the high and low tide called?  The difference between the high and low tide are called tidal ranges.